Corneal implant and method of manufacture

ABSTRACT

Prosthetic implants designed to be implanted in the cornea for modifying the cornea curvature and altering the corneal refractive power for correcting myopia, and myopia with astigmatism, such implants formed of a micro-porous hydrogel material.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/385,103, filed Aug. 27, 1999, now U.S. Pat. No. 6,361,560 which is acontinuation-in-part of U.S. patent application Ser. No. 09/219,594,filed Dec. 23, 1998 now U.S. Pat. No. 6,102,946.

FIELD OF THE INVENTION

The field of this invention relates to prosthetic implants designed tobe implanted in the cornea for modifying the cornea curvature andaltering the corneal refractive power for correcting myopia, hyperopia,astigmatism, and presbyopia, and, in addition, to such implants formedof a micro-porous hydrogel material.

BACKGROUND OF THE INVENTION

It is well known that anomalies in the shape of the eye can be the causeof visual disorders. Normal vision occurs when light that passes throughand is refracted by the cornea, the lens, and other portions of the eye,and converges at or near the retina. Myopia or near-sightedness occurswhen the light converges at a point before it reaches the retina and,conversely, hyperopia or far-sightedness occurs when the light convergesa point beyond the retina. Other abnormal conditions include astigmatismwhere the outer surface of the cornea is irregular in shape and effectsthe ability of light to be refracted by the cornea. In addition, inpatients who are older, a condition called presbyopia occurs in whichthere is a diminished power of accommodation of the natural lensresulting from the loss of elasticity of the lens, typically becomingsignificant after the age of 45.

Corrections for these conditions through the use of implants within thebody of the cornea have been suggested. Various designs for suchimplants include solid and split-ring shaped, circular flexible bodymembers and other types of ring-shaped devices that are adjustable.These implants are inserted within the body of the cornea for changingthe shape of the cornea, thereby altering the its refractive power.

These types of prostheses typically are implanted by first making atunnel and/or pocket within the cornea which leaves the Bowman'smembrane intact and hence does not relieve the inherent natural tensionof the membrane.

In the case of hyperopia, the corneal curvature must be steepened, andin the correction of myopia, it must be flattened. The correction ofastigmatism can be done by flattening or steepening various portions ofthe cornea to correct the irregular shape of the outer surface. Bi-focalimplants can be used to correct for presbyopia.

It has been recognized that desirable materials for these types ofprostheses include various types of hydrogels. Hydrogels are considereddesirable because they are hydrophilic in nature and have the ability totransmitting fluid through the material. It has been accepted that thistransmission of fluid also operates to transmit nutrients from thedistal surface of the implant to the proximal surface for providingproper nourishment to the tissue in the outer portion of the cornea.

However, while hydrogel lenses do operate to provide fluid transferthrough the materials, it has been found that nutrient transfer isproblematic because of the nature of fluid transfer from cell-to-cellwithin the material. Nutrients do not pass through the hydrogel materialwith the same level of efficacy as water. Without the proper transfer ofnutrients, tissue in the outer portion of the cornea will die causingfurther deterioration in a patient's eyesight.

Thus, there is believed to be a demonstrated need for a material forcorneal implants that will allow for the efficacious transmission ofnutrients from the inner surface of a corneal implant to the outersurface, so that tissue in the outer portion of the cornea is properlynourished. There is also a need for a more effective corneal implant forsolving the problems discussed above.

DESCRIPTION OF THE PRIOR ART Summary of the Invention

The present invention is directed to a corneal implant formed of abiocompatible, permeable, micro-porous hydrogel with a refractive indexsubstantially similar to the refractive index of the cornea. The device,when placed under a lamellar dissection made in the cornea (such as acorneal flap), to relieve tension of Bowman's membrane, alters the outersurface of the cornea to correct the refractive error of the eye. Byrelieving the pressure and subsequent implantation of the device, thepressure points which typically are generated in present cornealsurgeries are eliminated, and hence reduced risk to patients ofextrusion of implants.

The implant is preferably generally circular in shape and is of a sizegreater than the size of the pupil in normal or bright light, and canspecifically be used to correct hyperopia, myopia, astigmatism, and/orpresbyopia. Due to the complete non-elastic nature of the cornealtissue, it is necessary to place the implant in the cornea with Bowman'smembrane compromised, such as through a corneal lamellar dissection, toprevent extrusion of the implant from the cornea over the lifetime ofthe implant. Extrusion is undesirable because it tends to cause clinicalcomplications and product failure.

Preferably, for the correction of hyperopia, the implant is formed intoa meniscus-shaped disc with its anterior surface radius smaller(steeper) than the posterior surface radius, and with negligible edgethickness. This design results in a device that has a thickness ordimension between the anterior and posterior surfaces along the centralaxis greater than at its periphery. When such an implant is placed underthe corneal flap, the optical zone of the cornea is steepened and apositive optical power addition is achieved.

For the correction of myopia, the implant is shaped into a meniscus lenswith an anterior surface curvature that is flatter than the posteriorsurface. When the implant is placed concentrically on the stromal bedthe curvature of the anterior surface of the cornea in the optic zone isflattened to the extent appropriate to achieve the desired refractivecorrection.

For astigmatic eyes, implants are fabricated with a cylindrical additionalong one of the axes. This device can be oval or elliptical in shape,with a longer axis either in the direction of cylindrical power additionor perpendicular to it. The implant preferably has a pair of markerssuch as, for example, protrusions, indentations or other types of visualindicators, in the direction of the cylindrical axis to easily mark andidentify this direction. This indexing assists the surgeon in the properplacement of the implant under the flap with the correct orientationduring surgery to correct astigmatism in any axis.

For simple or compound presbyopia, the implant is made by modifying theradius of curvature in the central 1.5-3 mm, thereby forming amulti-focal outer corneal surface where the central portion of thecornea achieves an added plus power for close-up work. The base of animplant designed for compound presbyopia can have a design to alter thecornea to achieve any desired correction for the myopic, hyperopic, orastigmatic eye.

The material from which any one or more of these implants are made ispreferably a clear, permeably, microporous hydrogel with a water contentgreater than 40% up to approximately 90%. The refractive index should besubstantially identical to the refractive index of corneal tissue. Thepermeability of the material is effected through a network of irregularpassageways such as to permit adequate nutrient and fluid transfer toprevent tissue necrosis, but which are small enough to act as a barrieragainst the tissue ingrowth from one side of the implant to another.This helps the transmembrane tissue viability while continuing to makethe implant removable and exchangeable.

The refractive index of the implant material should be in the range of1.36-1.39, which is substantially similar to that of the cornea (1.376).This substantially similar refractive index prevents optical aberrationsdue to edge effects at the cornea-implant interface.

The microporous hydrogel material can be formed from at least one (andpreferably more) hydrophilic monomer, which is polymerized andcross-linked with at least one multi- or di-olefinic cross-linkingagent.

The implants described above can be placed in the cornea by making asubstantially circular lamellar flap using any commercially availablemicrokeratome. When the flap is formed, a hinge is preferably left tofacilitate proper alignment of the dissected corneal tissue after theimplant is placed on the exposed cornea.

The implants described above which can be used for correcting hyperopiaor hyperopia with astigmatism are preferably made into a disc shape thatis nominally about 4.5 mm in diameter and bi-meniscus in shape. Thecenter of the lens is preferably no greater than 50 micrometers thick.The edge thickness should be less than two keratocytes (i.e., about 15micrometers).

An improvement over the lenses described above for correcting myopiawith astigmatism includes forming a lens in the shape of a ring with oneor more portions in the center being solid and defining voids in thecenter section for shaping the astigmatic component by providing solidportions under the flatter meridian of the astigmatic myopic eye. Anexample of such a shape includes a ring with a rib extending across thecenter that is either squared off or rounded where it contacts the ring.Another example is a ring with one or more quadrants filled in, with theother ones forming voids. Other shapes can used to provide a solidportion under the flatter meridan.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention can be obtained from thedetailed description of exemplary embodiments set forth below, whenconsidered in conjunction with the appended drawings, in which:

FIG. 1 is a schematic illustration of a horizontal section of a humaneye;

FIG. 2 is a schematic illustration of an eye system showing adjustmentof the cornea to steepen the corneal slope to correct for hyperopia;

FIG. 3 is a schematic illustration of an eye system showing adjustmentof the cornea to flatten the corneal slope to correct for myopia;

FIGS. 4a and 4 b are sectional and plan views of a solid corneal implantfor correcting hyperopia;

FIGS. 5a and 5 b are sectional and plan views of a solid corneal implantfor correcting myopia;

FIGS. 6a and 6 b are sectional and plan views of ring-shaped cornealimplant for correcting myopia;

FIGS. 7a and 7 b are schematic representations of a lamellar dissectomy,with FIG. 7b showing in particular the portion of the dissected corneabeing connected through a hinge to the intact cornea;

FIG. 8 is a schematic representations of a cornea in which an implanthas been implanted for a hyperopic correction;

FIGS. 9 and 10 are schematic representations of a cornea in which solidand ring-shaped implants, respectively, have been implanted lamellar fora myopic correction;

FIGS. 11a, 11 b, and 11 c are plan and sectional views of an implantuseful for correcting astigmatism where two axes have different diopterpowers;

FIGS. 12a, 12 b, and 12 c are plan and sectional views of an secondimplant for correcting astigmatism where the implant is elliptical inshape;

FIG. 13 is a plan view of an implant with a pair of tabs used toidentify an axis for astigmatic correction;

FIG. 14 is a plan view of a second implant for astigmatic correctionwhere indentations are used instead of tabs;

FIGS. 15 and 16 are schematic representations showing implants with tabsorientated along the astigmatic axis for correcting astigmatism;

FIG. 17 is a sectional view of a corneal implant shaped to correct forcompound presbyopia with an additional power in the center of an implantfor correcting hyperopia;

FIG. 18 is a sectional view of another corneal implant shaped to correctfor compound presbyopia with additional power in the center of animplant for correcting myopia;

FIG. 19 is a sectional view of a corneal implant with additional powerin the center for correcting simple presbyopia;

FIG. 20a is a schematic representation of a corneal implant for anastigmatic correction with a central power add for correctingpresbyopia, showing in particular a pair of tabs for proper alignment ofthe lens;

FIG. 20b is a schematic representation of a another corneal implant witha center power add for non-astigmatic correction, which shows inparticular a steep transition between the central add and the remainderof the implant;

FIGS. 21a and 21 b are schematic representations showing the use of alamellar dissection for implanting a lens of the type shown in FIG. 20b;and

FIGS. 22 and 23 are schematic representations of several lenses usefulfor correcting myopia with astigmatism formed in the shape of a ringwith a rib extending across the center of the lens; and

FIG. 24 is another schematic representation of another lens forcorrecting myopia with astigmatism where the ring-shaped lens has onequadrant that is solid, while the rest of the center portion forms avoid.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring first to FIG. 1 of the drawings, a schematic representation ofthe globe of the eye 10 is shown, which resembles a sphere with ananterior bulged spherical portion 12 that represents the cornea. The eye10 is made up of three concentric coverings that enclose the varioustransparent media through which light must pass before reaching thelight sensitive retina 14.

The outer-most covering is a fibrous protective portion that includes aposterior layer which is white and opaque, called the sclera 16, whichis sometimes referred to as the white of the eye where it is visiblefrom the front. The anterior ⅙th of this outer layer is the transparentcornea 12.

A middle covering is mainly vascular and nutritive in function and ismade up of the choroid 18, the ciliary 20 and the iris 22. The choroidgenerally functions to maintain the retina. The ciliary muscle 21 isinvolved in suspending the lens 24 and accommodating the lens. The iris22 is the most anterior portion of the middle covering of the eye and isarranged in a frontal plane. The iris is a thin circular disccorresponding to the diaphragm of a camera, and is perforated near itscenter by a circular aperture called the pupil 26. The size of the pupilvaries to regulate the amount of light that reaches the retina 14. Itcontracts also to accommodate, which serves to sharpen the focus bydiminishing spherical aberrations. The iris 22 divides the space betweenthe cornea 12 and the lens 24 into an anterior chamber 28 and posteriorchamber 30.

The inner-most covering is the retina 14, consisting of nerve elementswhich form the true receptive portion for visual impressions that aretransmitted to the brain. The vitreous 32 is a transparent gelatinousmass which fills the posterior ⅘ths the globe 10. The vitreous supportsthe ciliary body 20 and the retina 14.

Referring to FIG. 2 of the drawings, the globe of an eye 10 is shown ashaving a cornea 12 with a normal curvature represented by a solid line34. For people with normal vision, when parallel rays of light 36 passthrough the corneal surface 34, they are refracted by the cornealsurfaces to converge eventually near the retina 14 (FIG. 1). The diagramof FIG. 2 discounts, for the purposes of this discussion, the refractiveeffect of the lens or other portions of the eye. However, as depicted inFIG. 2, when the eye is hyperopic the rays of light 36 are refracted toconverge at a point 38 behind the retina.

If the outer surface of the cornea 12 is caused to steepen, as shown bydotted lines 40, such as through the implantation of a corneal implantof an appropriate shape as discussed below, the rays of light 36 arerefracted from the steeper surface at a greater angle as shown by dottedlines 42, causing the light to focus at a shorter distance, such asdirectly on the retina 14.

FIG. 3 shows a similar eye system to that of FIG. 2 except that thenormal corneal curvature causes the light rays 36 to focus at a point 44in the vitreous which is short of the retinal surface. This is typicalof a myopic eye. If the cornea is flattened as shown by dotted lines 46through the use of a properly-shaped corneal implant, light rays 36 willbe refracted at a smaller angle and converge at a more distant pointsuch as directly on the retina 14 as shown by dotted lines 48.

A hyperopic eye of the type shown in FIG. 2 can be corrected byimplanting an implant 50 having a shape as shown in FIGS. 4a, 4 b. Theimplant 50 is in the shape of a meniscus lens with an outer surface 52that has a radius of curvature that is smaller than the radius ofcurvature of the inner surface 54. When a lens of this type is implantedusing the method discussed below, it will cause the outer surface of thecornea to become steeper in shape as shown by reference numeral 40 inFIG. 2, correcting the patient's vision so that light entering the eyewill converge on the retina as shown by the dotted lines 42 in FIG. 2.

The lens 50 shown in FIGS. 4a and 4 b is formed with a bi-meniscusshape, with the anterior and posterior surfaces having different radiiof curvature.

The anterior surface has a greater radius than the posterior surface.The lens 50 preferably has a nominal diameter of about 4.5 mm. Thecenter of the lens is preferably no greater than 50 micrometers thick toenhance the diffusion characteristics of the material from which thelens is formed, which allows for more effective transmission ofnutrients through the lens material and promotes better health of theanterior corneal tissue. The outer edge of the lens 50 has a thicknessthat is less than the dimensions of two keratocytes (i.e., about 15micrometers) juxtaposed side-by-side, which are the fixed flattenedconnective tissue cells between the lamellae of the cornea. An edgethickness as specified prevents stacking and recruitment of keratocytesin the lens material so that keratocyte stacking and recruitment doesnot take place. This in turn eliminates unorganized collagen that formsundesirable scar tissue and infiltrates the lens, which tends tocompromise the efficacy of the lens.

On the other hand, in order to cure myopia, an implant 56 having theshape shown in FIGS. 5a, 5 b, can be used where an outer surface 58 isflatter or formed with a larger radius than that of the inner surface 60which is formed with a radius of curvature substantially identical tothat of the corneal stroma bed generated by the lamellar dissectiondescribed below. The implant 56 has a transition zone 62 formed betweenthe outer and inner surfaces 58, 60, which is outside of the opticalzone. In this way, the curvature of the outer surface of the cornea, asshown in FIG. 3, is flattened to an extent appropriate to achieve theproper refractive correction desired so that light entering the eye willconverge on the retina as shown in FIG. 3.

Alternatively, instead of using a solid implant as shown in FIGS. 5a, 5b, for correcting myopia, a ring 64 of the type shown in FIGS. 6a, 6 bcould be used. This ring has substantially the same effect as theimplant shown in FIGS. 5a, 5 b, by flattening the outer surface of thecornea shown in FIG. 3. The ring 64 has a center opening 66 that ispreferably larger than the optical zone so as not to cause sphericalaberrations in light entering the eye.

Implants of the type shown in FIGS. 4, 5 and 6 can be implanted in thecornea using a lamellar dissectomy shown schematically in FIGS. 7a, 7 b.In this procedure, a keratome (not shown) is used in a known way to cuta portion of the outer surface of the cornea 12 along dotted lines 68 asshown in FIG. 7a. This type of cut is used to form a corneal flap 70shown in FIG. 7b, which remains attached to the cornea 12 through whatis called a hinge 72. The hinge 72 is useful for allowing the flap 70 tobe replaced with the same orientation as before the cut.

As is also known in the art, the flap is cut deeply enough to dissectthe Bowman's membrane portion of the cornea, such as in keratome surgeryor for subsequent removal of the tissue by laser or surgical removal. Acorneal flap of 100 to 200 microns, typically 160 to 180 microns, willbe made to eliminate the Bowman's membrane tension. This reduces thepossibility of extrusion of the implants due to pressure generatedwithin the cornea caused by the addition of the implant. Implants of thetype shown in FIGS. 4, 5 and 6 are shown implanted in corneas in FIGS.8, 9 and 10, respectively, after the flap has been replaced in itsnormal position. These figures show the corrected shape for the outersurface of the cornea as a result of implants of the shapes described.

Implants can also be formed with a cylindrical addition in one axis ofthe lens in order to correct for astigmatism, as shown in the implantsin FIGS. 11-16. Such implants can be oval or elliptical in shape, whichthe longer axis either in the direction of cylindrical power addition orperpendicular to it. For example, the implant can be circular as shownin FIG. 11a where the implant 72 has axes identified as x, y. In thecase of a circular implant 72, the axes of the implant have differentdiopter powers as shown in FIGS. 11b and 11 bc, which arecross-sectional views of the implant 72 along the x and y axes,respectively. The different thicknesses of the lenses in FIGS. 11b and11 c illustrate the different diopter powers along these axes.

Alternatively, as shown in FIG. 5a, an astigmatic implant 74 can be ovalor elliptical in shape. The implant 74 also has axes x, y. As shown inthe cross-sectional views of the implant 74 in FIGS. 12b, 12 c, alongthose two axes, respectively, the implant has different diopter powersas shown by the different thicknesses in the figures.

Because implants of the type identified by reference numeral 72, 74 arerelatively small and transparent, it is difficult for the surgeon tomaintain proper orientation along the x and y axes. In order to assistthe surgeon, tabs 76 a, 76 b or indentations 78 a, 78 b are used toidentify one or the other of the axis of the implant to maintain properalignment during implantation. This is shown in FIGS. 15, 16 where, forexample, indentations 76 a, 76 b, are aligned with axis x which has beendetermined as the proper axis for alignment in order to effect theastigmatic correction. Alternatively, other types of markers could beused such as visual indicators such as markings on or in the implantsoutside of the optical zone.

Referring to FIGS. 17-21, implants with presbyopic corrections areshown. In FIG. 17, an compound implant 80 is shown, which is appropriatefor hyperopic correction, which has an additional power section 82 inthe center. As shown, the implant 82 has anterior and posteriorcurvatures similar to those in FIGS. 4a, 4 b, in order to correct forhyperopia. In FIG. 18, a central power add 84 is formed on anothercompound implant 86, which has a base shape similar to the one shown inFIGS. 5a, 5 b, and is appropriate for a myopic correction. In FIG. 19, acentral power portion 88 is added to an simple planal implant 90 whichhas outer and inner surfaces of equal radii, which does not add anycorrection other than the central power.

The central power add portions 82, 84, and 88 are preferably within therange of 1.5-3 mm in diameter, most preferably 2 mm, and which provide amulti-focal outer corneal surface where the central portion of thecornea achieves an added plus power for close-up work. In addition tothe based device having no correction, or corrections for hyperopia ormyopia, the base device can have a simple spherical correction forastigmatism as shown in FIG. 20a, where a central power add 92 is addedto an implant 94 similar to the one shown in FIG. 11a, which alsoincludes tabs 76 a, 76 b.

As shown in FIG. 20b in order to enhance the acuity of a presbyopicimplant, a transition zone 96 can be formed around the central power add98 for implant 100. This transition zone 96 is a sharp zone change inpower from central added power to peripheral base power and is anchoredover a radial distance 0.5 to 0.2 mm start to from the end of thecentral zone.

Implantation of the device shown in FIG. 20b, is illustrated in FIGS.21a, 21 b, where a flap 102 formed through a lamellar dissectomy isshown pulled back in FIG. 21a so that the implant 100 can be positioned,and then replaced as shown in FIG. 21b for the presbyopic correction. Asshown, the formation of a sharp transition 96 on the implant 100provides a well defined central power after implantation is complete.

FIGS. 22 and 23 illustrate lenses 166, 168, respectively, which areuseful for correcting myopia with astigmatism. As shown, these lensesare ring-shaped, similar to the one in FIGS. 6a, 6 b. However, thelenses 166, 168 include rib sections 166 a, 168 a, respectively, whichextend across the center of each lens and define voids between the ribsand the outer periphery of the lenses. These solid rib sections shapethe astigmatic component by providing solid portions under the flattermeridian of the astigmatic myopic eye, when these flatter portions arelocated above the ribs. The ribs 166 a, 168 a can be formed in anysuitable shape such as, by way of example, the rib 166 a being squaredoff as shown in FIG. 22 or the rib 168 a being rounded s shown in FIG.23, where they contact their respective rings.

Another example of a design for correcting myopia with astigmatism is alens 170 as shown in FIG. 24, which is also ring-shaped but has one itsquadrants 170 a filled in. This lens can be used where the flatterportion of an astigmatic eye is located in a position where the quadrantcan be located beneath the flatter portion. The solid portion of thelens will tend to raise the flattened portion so that a smooth roundedouter surface is formed. As can readily be appreciated, lenses can beformed with solid portions located in any number of places where theycan positioned under the flattened portion of an astigmatic eye toachieve the same end.

The implants described above are preferably formed of a microporoushydrogel material in order to provide for the efficacious transmissionof nutrients from the inner to the outer surface of the implants. Thehydrogels also preferably have micropores in the form of irregularpassageways, which are small enough to screen against tissue ingrowth,but large enough to allow for nutrients to be transmitted. Thesemicroporous hydrogels are different from non-microporous hydrogelsbecause they allow fluid containing nutrients to be transmitted betweenthe cells that make up the material, not from cell-to-cell such as innormal hydrogel materials. Hydrogels of this type can be formed from atleast one, and preferably more, hydrophillic monomer which ispolymerized and cross-linked with at least one multi- or di-olefiniccross-linking agent.

An important aspect of the materials of the present invention is thatthe microporous hydrogel have micropores in the hydrogel. Suchmicropores should in general have a diameter ranging from 50 Angstromsto 10 microns, more particularly ranging from 50 Angstroms to 1 micron.A microporous hydrogel in accordance with the present invention can bemade from any of the following methods.

Hydrogels can be synthesized as a zero gel by ultraviolet or thermalcuring of hydrophillic monomers and low levels of cross-linking agentssuch as diacrylates and other UV or thermal initiators. These lightlycross-linked hydrogels are then machined into appropriate physicaldimensions and hydrated in water at elevated temperatures. Upon completehydration, hydrogel prosthesis are flash-frozen to temperatures belownegative 40 degree C., and then gradually warmed to a temperature ofnegative 20 degree C. to negative 10 degree C. and maintained at thesame temperature for some time, typically 12 to 48 hours, in order togrow ice crystals to larger dimensions to generate the porous structurevia expanding ice crystals. The frozen and annealed hydrogel is thenquickly thawed to yield the microporous hydrogel device. Alternatively,the hydrated hydrogel device can be lyophilized and rehydrated to yielda microporous hydrogel.

Still further, the microporous hydrogel can also be made by startingwith a known formulation of monomers which can yield a desiredcross-linked hydrogel, dissolving in said monomer mixture a lowmolecular weight polymer as a filler which is soluble in said mixtureand then polymerizing the mixture. Resulted polymer is converted intothe required device shape and then extracted with an appropriate solventto extract out the filled polymer and the result in a matrix hydrated toyield a microporous device.

Still further and alternatively, microporous hydrogels can also be madeby any of the above methods with the modification of adding an adequateamount of solvent or water to give a pre-swollen finished hydrogel,which can then be purified by extraction. Such formulation can bedirectly cast molded in a desired configuration and do not requiresubsequent machining processes for converting.

I claim:
 1. A corneal implant for correcting myopia with astigmatism,comprising: (a) a body formed of an optically clear, biocompatible,material having an index of refraction substantially the same as that ofcorneal tissue; (b) the body having an outer ring-shaped portion that issolid and defines an opening; and (c) the body including a second solidportion that fills in a portion of the opening, the second solid portionbeing located in a position to be implanted under a flattened portion ofan astigmatic eye.
 2. The implant of claim 1, wherein the body has anouter edge with a thickness of the edge being less than about 15micrometers.
 3. The implant of claim 1 or 2, wherein the second solidportion comprises a rib extending across the center of the opening. 4.The implant of claim 3, wherein the rib is squared off where it joinsthe ring-shaped portion.
 5. The implant of claim 4, wherein the rib isrounded where it joins the ring-shaped portion.
 6. The implant of claim1, wherein the biocompatible, material is a microporous hydrogel.
 7. Theimplant of claim 6, wherein the microporous hydrogel has a water contentgreater than 40% up to approximately 90%.
 8. The implant of claim 6,wherein the microporous hydrogel has passageways to permit nutrient andfluid transfer, said passageways being small enough to act as a barrieragainst tissue growth.
 9. The implant of claim 6, wherein themicroporous hydrogel is made from at least one hydrophilic monomer whichis polymerized and cross-linked with at least one multi- or di-olefiniccross-linking agent.
 10. The implant of claim 1, wherein the body has anouter edge with a thickness of the edge being less than about twokeratocytes.
 11. A method of implanting a corneal implant for correctingmyopia with astigmatism, comprising the steps of: (a) cutting away aportion of the outer surface of a cornea; (b) implanting a lens on theexposed surface of the cornea with a body formed of an optically clear,biocompatible, material having an index of refraction substantially thesame as that of corneal tissue, the body having an outer ring-shapedportion that is solid and defines an opening, the body including asecond solid portion that fills in a portion of the opening, the secondsolid portion being located in a position to be implanted under aflattened portion of an astigmatic eye; and (c) replacing the portion ofthe cornea that was cut away.
 12. The method of claim 11, wherein thebody has an outer edge with a thickness of the edge being less thanabout 15 micrometers.
 13. The implant of claim 11, wherein thebiocompatible, material is a microporous hydrogel.
 14. The implant ofclaim 13, wherein the microporous hydrogel has a water content greaterthan 40% up to approximately 90%.
 15. The implant of claim 13, whereinthe microporous hydrogel has passageways to permit nutrient and fluidtransfer, said passageways being small enough to act as a barrieragainst tissue growth.
 16. The implant of claim 13, wherein themicroporous hydrogel is made from at least one hydrophilic monomer whichis polymerized and cross-linked with at least one multi- or di-olefiniccross-linking agent.
 17. The implant of claim 11, wherein the body hasan outer edge with a thickness of the edge being less than about twokeratocytes.
 18. A corneal implant for correcting myopia, comprising:(a) a body formed of an optically clear, biocompatible, material havingan index of refraction substantially the same as that of corneal tissue;(b) the body having an outer ring-shaped portion that is solid anddefines an opening; and (c) the body having an outer edge with an outeredge with a thickness of the edge being less than about 15 micrometers.19. The implant of claim 18, wherein the body is generally circular inshape.
 20. The implant of claim 18, wherein the biocompatible, materialis a microporous hydrogel.
 21. The implant of claim 20, wherein themicroporous hydrogel has a water content greater than 40% up toapproximately 90%.
 22. The implant of claim 20, wherein the microporoushydrogel has passageways to permit nutrient and fluid transfer, saidpassageways being small enough to act as a barrier against tissuegrowth.
 23. The implant of claim 20, wherein the microporous hydrogel ismade from at least one hydrophilic monomer which is polymerized andcross-linked with at least one multi- or di-olefinic cross-linkingagent.
 24. The implant of claim 18, wherein the body has an outer edgewith a thickness of the edge being less than about two keratocytes. 25.A method of implanting a corneal implant for correcting myopia,comprising the steps of: (a) cutting away a portion of the outer surfaceof a cornea; (b) implanting a lens on the exposed surface of the corneawith a body formed of an optically clear, biocompatible, material havingan index of refraction substantially the same as that of corneal tissue,the body having an outer ring-shaped portion that is solid and definesan opening; and (c) replacing the portion of the cornea that was cutaway.
 26. The method of claim 25, wherein the body has an outer edgewith a thickness of the edge being less than about 15 micrometers. 27.The implant of claim 25, wherein the biocompatible, material is amicroporous hydrogel.
 28. The implant of claim 27, wherein themicroporous hydrogel has a water content greater than 40% up toapproximately 90%.
 29. The implant of claim 27, wherein the microporoushydrogel has passageways to permit nutrient and fluid transfer, saidpassageways being small enough to act as a barrier against tissuegrowth.
 30. The implant of claim 27, wherein the microporous hydrogel ismade from at least one hydrophilic monomer which is polymerized andcross-linked with at least one multi- or di-olefinic cross-linkingagent.
 31. The implant of claim 25, wherein the body has an outer edgewith a thickness of the edge being less than about two keratocytes.